README.md: fix err check in example

Fixes #3

Reported-by: @tomparkin
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

README.md

@@ -36,7 +36,7 @@ import (
 func main() {
 	// get clock ticks, this will return the same as C.sysconf(C._SC_CLK_TCK)
 	clktck, err := sysconf.Sysconf(sysconf.SC_CLK_TCK)
-	if err != nil {
+	if err == nil {
 		fmt.Printf("SC_CLK_TCK: %v\n", clktck)
 	}
 }